Titanium containing oxide has been developed as a negative electrode material for nonaqueous electrolyte battery such as a lithium-ion secondary battery. The battery using the titanium containing oxide can be carried out rapid charging and discharging stably. Further, it has a long life as compared with a battery using a carbon material as a negative electrode material. However, the battery using the titanium containing oxide has a problem such that the energy density is lower.
One of the reasons that the energy density is lower in the battery using the titanium containing oxide is that an electrode comprising titanium containing oxide has a higher potential compared with an electrode comprising a carbon material. For example, the electrode comprising the titanium containing oxide causes insertion/release of lithium ion about 1.5 V (vs Li/Li+). The term “V (vs Li/Li+)” is mean to a potential relative to metallic lithium.
If the negative electrode potential is lowered, the energy density of the battery is improved. However, this is substantially difficult because of the following reasons.
When lithium ion inserts into or releases from the titanium containing oxide, titanium in the oxide is reduced or oxidized. In this case, the titanium is changed from Ti4+ to Ti3+ or reversely. The potential of the electrode is resulted from the oxidation-reduction reaction of the titanium. Thus, the potential of the electrode is electrochemically restricted. Further, there is a fact that the electrode comprising the titanium containing oxide can be carried out rapid charging and discharging stably because the potential is around 1.5 V (vs Li/Li+). Therefore, it is difficult to decrease the potential of the electrode.
Another reason that the energy density is lower in the battery using the titanium containing oxide is that the capacity per mass of the electrode comprising the titanium containing oxide is lower as compared with that of the electrode comprising a carbon material. For example, the theoretical capacity of a titanium dioxide having anatase structure is about 165 mAh/g. The theoretical capacity of a lithium titanium composite oxide having spinel structure (Li4Ti5O12) is about 170 mAh/g. On the other hand, the theoretical capacity of a carbon material such as graphite is 385 mAh/g or more.
One of the reasons that the capacity of the titanium containing oxide is lower is that there are not many sites for lithium ion to insert in a crystal structure. Another reason is that lithium ion is easily stabilized in the crystal structure.
Recently, a monoclinic titanium dioxide has attracted attention. The theoretical capacity of the monoclinic titanium dioxide reaches about 330 mAh/g. However, an effective capacity of the monoclinic titanium dioxide is remarkably lower than the theoretical capacity.